1. Field of the Invention
The present invention relates to a device for controlling torsional stiffness of a power steering system, particularly a device for controlling torsional stiffness of a power steering system that improve stability in operation of a steering wheel by changing stiffness of a torsion bar according to driving conditions of a vehicle.
2. Description of Related Art
In the related art, a power steering system that supplements operational force that is applied to the steering wheel by a driver has been widely used for a steering system of vehicles and a hydraulic power steering system that supplements operational force that is applied to the steering wheel by generating hydraulic pressure has been generally used in the power steering system.
The hydraulic power steering system includes, a power unit having a valve unit composed of a hydraulic pump generating hydraulic pressure, which is power source, a pressure control valve, and a flow control valve, an actuating unit using an actuating cylinder to generates auxiliary steering force by converting the hydraulic pressure generated by the hydraulic pump into mechanical force, and a control unit controlling the operational direction and condition of the actuating cylinder by opening/closing a flow channel connected to the actuating unit and changing the flow channel by operation of the steering wheel.
The control unit includes, an input shaft that is connected with the steering wheel and rotates integrally with the steering wheel, a pinion gear that is engaged with a rack bar of a steering gear box to move the rack bar to the left and right, a torsion bar that connects the input shaft with the pinion gear such that they integrally rotate, and a control valve that appropriately changes the flow channel to supply the hydraulic pressure supplied from the hydraulic pump to the actuating cylinder by the operation of steering wheel.
As a driver operates the steering wheel, the input shaft is rotated by the operation of the steering wheel and the rotation of the input shaft is transmitted to the pinion gear, such that the pinion gear moves the rack bar while rotating and the wheels turn. In this operation, the control valve is changed in the amount of opening/closing by the relative positional difference between the input shaft and the pinion gear and the amount of flow through the control valve is controlled. Accordingly, hydraulic pressure is supplied to the actuating cylinder and supplements the operational force applied to the steering wheel by the driver.
The operational characteristics of the control valve are changed by the stiffness of the torsion bar in the power steering system in the related art as described above. Therefore, when the stiffness of the torsion bar is small, the relative positional difference between the input shaft and the pinion shaft becomes large, even if the driver applies small force to operate the steering wheel, such that the flow rate and hydraulic pressure through the control valve increases and the auxiliary force for the steering wheel transmitted through the actuating cylinder increases, which decreases handing force for the steering wheel. On the contrary, when the stiffness of the torsion bar is large, the relative positional difference between the input shaft and the pinion shaft is small, even if the driver applies large force to operate the steering wheel, the flow rate and hydraulic pressure through the control valve decreases and the auxiliary force for the steering wheel transmitted through the actuating cylinder decreases, which increases handling force for the steering wheel.
However, in the power steering system in the related art as described above, because the stiffness of the torsion bar is fixed to a specific value in design, it is limited to achieve desired characteristics of hydraulic torque.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.